By Carl Zimmer
Your body is home to about 100 trillion bacteria and other microbes, collectively known as your microbiome. Naturalists first became aware of our invisible lodgers in the 1600s, but it wasn’t until the past few years that we’ve become really familiar with them.
This recent research has given the microbiome a cuddly kind of fame. We’ve come to appreciate how beneficial our microbes are, breaking down our food, fighting off infections and nurturing our immune system. It’s a lovely, invisible garden we should be tending for our own well-being.
But in the journal BioEssays, a team of scientists has raised a creepier possibility. Perhaps our menagerie of germs is also influencing our behaviour in order to advance its own evolutionary success — giving us cravings for certain foods, for example.
Maybe the microbiome is our puppet master.
“One of the ways we started thinking about this was in a crime-novel perspective,” said Carlo C Maley, an evolutionary biologist at the University of California, San Francisco, and a co-author of the new paper. “What are the means, motives and opportunity for the microbes to manipulate us? They have all three.”
The idea that a simple organism could control a complex animal may sound like science fiction. In fact, there are many well-documented examples of parasites controlling their hosts.
Some species of fungi, for example, infiltrate the brains of ants and coax them to climb plants and clamp onto the underside of leaves. The fungi then sprout out of the ants and send spores showering onto uninfected ants below.
How parasites control their hosts remains mysterious. But it looks as if they release molecules that can directly or indirectly influence their host’s brain.
Our microbiome has the biochemical potential to do the same thing. In our guts, bacteria make some of the same chemicals that our neurons use to communicate with one another, such as dopamine and serotonin. And the microbes can deliver these neurological molecules to the dense web of nerve endings that line the gastrointestinal tract.
A number of recent studies have shown that gut bacteria can use these signals to alter the biochemistry of the brain. Compared with ordinary mice, those raised free of germs behave differently in a number of ways. They are more anxious, for example.
Adding certain species of bacteria to a normal mouse’s microbiome reveals other ways in which they can influence behaviour. Some bacteria lower stress levels in the mouse. When scientists sever the nerve relaying signals from the gut to the brain, this stress-reducing effect disappears.
Some experiments suggest that bacteria also influence the way their hosts eat. Germ-free mice develop more receptors for sweet flavours in their intestines. Scientists have also found that bacteria can alter levels of hormones that govern appetite in mice.
Maley and his colleagues argue that our eating habits create a strong motive for microbes to manipulate us. “From the microbe’s perspective, what we eat is a matter of life and death,” Maley said.
Different species of microbes thrive on different kinds of food. If they can prompt us to eat more of the food they depend on, they can multiply. Microbial manipulations might fill in some of the puzzling holes in our understanding of food cravings, Maley said. Scientists have tried to explain food cravings as the body’s way to build up a supply of nutrients after deprivation, or as addictions, much like those for tobacco and cocaine.
But both explanations fall short. Take chocolate: Many people crave it fiercely, but it isn’t an essential nutrient. And chocolate doesn’t drive people to increase the dose to get a high. “You don’t need more chocolate at every sitting to enjoy it,” Maley said.
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